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Register a new userAnisotropic quasiparticle coherence in nematic BaFe$_2$As$_2$ studied with strain-dependent ARPES
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The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in BaFe$_2$As$_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $Delta epsilon_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hunds metal picture. We also discuss the influence of orbital mixing.
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An instrumentation problem with the signal acquisition at high frequencies was discovered and we no longer believe that the experimental data presented in the manuscript, showing a frequency enhancement of the elastoresistivity, are correct. After correcting the problem, the elastoresistivity data is frequency independent in the range investigated. Therefore, the authors have withdrawn this submission. We would like to thank Alex Hristov, Johanna Palmstrom, Josh Straquadine and Ian Fisher (Stanford) for the kind discussions and assistance we received which helped us identify these problems.
In several Fe-based superconductors, slight $C_4$ symmetry breaking occurs at $T^*$, which is tens of Kelvin higher than the structural transition temperature $T_S$. In this hidden nematic state at $T_S<T<T^*$, the orthorhombicity is tiny [$phi=(a-b)/(a+b) ll 0.1$%], but clear evidences of bulk phase transition have been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the antiferro wavevector ${bf q}=(0,pi)$ at $T=T^*$, by which the characteristic phenomena below $T^*$ are satisfactorily explained. This AFB order originates from the inter-orbital nesting between the $d_{xy}$-orbital hole-pocket and the electron-pocket, and this inter-orbital bond order naturally explains the pseudogap, band-folding, and tiny nematicity that is linear in $T^*-T$. The hidden AFB order explains key experiments in both BaFe$_2$As$_2$ and NaFeAs, but it is not expected to occur in FeSe because of the absence of the $d_{xy}$-orbital hole-pocket.
The BaNi$_2$As$_2$ compound is investigated using both the angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and combined computational scheme of local density approximation together with dynamical mean-field theory (LDA+DMFT). For more realistic comparison of LDA+DMFT spectral functions with ARPES data we take into account several experimental features: the photoemission cross-section, the experimental energy and angular resolutions and the photo-hole lifetime effects. In contrast to isostructural iron arsenides the BaNi$_2$As$_2$ within LDA+DMFT appears to be weakly correlated (effective mass enhancement about $1.2$). This dramatic reduction of the correlation strength comes from the increase of 3d-orbital filling, when going from Fe to Ni, together with rather large bare Ni-3d LDA bandwidth. Nevertheless, even weakened electron correlations cause remarkable reconstruction of the bare BaNi$_2$As$_2$ LDA band structure and corresponding LDA+DMFT calculations provide better agreement with ARPES than just renormalized LDA results.
We discuss the results of $^{75}$As Nuclear Quadrupole Resonance (NQR) and muon spin relaxation measurements in AFe$_2$As$_2$ (A= Cs, Rb) iron-based superconductors. We demonstrate that the crossover detected in the nuclear spin-lattice relaxation rate $1/T_1$ (around 150 K in RbFe$_2$As$_2$ and around 75 K in CsFe$_2$As$_2$), from a high temperature nearly localized to a low temperature delocalized behaviour, is associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the NQR spectra as well as an increase in the muon spin relaxation rate. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is associated with a charge disproportionation at the Fe sites induced by competing Hunds and Coulomb couplings. In RbFe$_2$As$_2$ around 35 K, far below that crossover temperature, we observe a peak in the NQR $1/T_1$ which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order.
We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ above and below its superconducting transition temperature $T_c=20$ K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector $(1,0,0)$, the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at $(1,0,0)$ and $(1,0,1)$. If the resonance energy is directly associated with the superconducting gap energy $Delta$, then $Delta$ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.
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